1. Field of the Invention
The present invention relates generally to the selective modification and control of a patient""s body temperature. More particularly, the present invention provides methods and apparatus for treating hypothermia or hyperthermia by inserting a catheter into a blood vessel of the patient and selectively controlling the temperature of a portion of the catheter within the blood vessel. Heat is transferred to or from blood flowing through the vessel and the patient""s body temperature may thereby be increased or decreased as desired.
2. Description of the Background Art
Under ordinary circumstances the thermoregulatory system of the human body maintains a near constant temperature of about 37xc2x0 C. (98.6xc2x0 F.). Heat lost to the environment is precisely balanced by heat produced within the body.
Hypothermia is a condition of abnormally low body temperature. Hypothermia can be clinically defined as a core body temperature of 35xc2x0 C. or less. Hypothermia is sometimes characterized further according to its severity. A body core temperature in the range from 32xc2x0 C. to 35xc2x0 C. is described as xe2x80x9cmildxe2x80x9d hypothermia, 30xc2x0 C. to 32xc2x0 C. is called xe2x80x9cmoderate,xe2x80x9d 24xc2x0 C. to 30xc2x0 C. is described as xe2x80x9csevere,xe2x80x9d and a body temperature less than 24xc2x0 C. constitutes xe2x80x9cprofoundxe2x80x9d hypothermia. Although the above ranges provide a useful basis for discussion, they are not absolutes and definitions vary widely in the medical literature.
Accidental hypothermia results when heat loss to the environment exceeds the body""s ability to produce heat internally. In many cases, thermoregulation and heat production are normal but the patient becomes hypothermic due to overwhelming environmental cold stress. This is a relatively common condition, often resulting from exposure to the elements. Hypothermia may also occur in patients exposed to mild cold stress whose thermoregulatory ability has been lessened due to injury or illness. For example, this type of hypothermia sometimes occurs in patients suffering from trauma or as a complication in patients undergoing surgery.
Hypothermia of either type is a dangerous condition which can have serious medical consequences. In particular, hypothermia interferes with the ability of the heart to pump blood. Hypothermia may be fatal for this reason alone. Additionally, low body temperature seriously interferes with the enzymatic reactions necessary for blood clotting. This sometimes results in bleeding that is very difficult to control, even when normal clotting factor levels are present. These effects and other adverse consequences of hypothermia lead to drastically increased mortality rates both among victims of trauma and in patients undergoing surgery.
Simple methods for treating hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. Even these simple methods may be effective if the hypothermia is not too severe. These simple methods are limited in their effectiveness however. Wrapping the patient in blankets ultimately depends on the patient""s own production of heat to rewarm his body. In even moderate cases of hypothermia, or in the case of an ill or injured patient, the patient may simply be too weak or exhausted to produce sufficient heat. Oral administration of a warm fluid requires that the patient be conscious and capable of swallowing the fluid. Since loss of consciousness occurs early in hypothermia, this method is also limited to moderate cases. Finally, immersion of the patient in a warm water bath is often simply impractical. For example, immersion of a patient undergoing surgery would obviously be undesirable. Furthermore, the immersion technique is time consuming and may be ineffective in that it requires the transmission of warmth from the patient""s skin surface into the body core before the benefit of the warmth can be realized.
For this reason, methods have been devised to allow for the direct warming of a patient""s blood. These methods involve removing blood from the patient, warming the blood in external warming equipment, and delivering the blood back into the patient. While such methods are much more effective than any of the simple methods previously described, they are disadvantageous for other reasons. First, the apparatus involved is quite cumbersome. Second, some danger is involved in even the temporary removal of significant quantities of blood from an already weakened patient. In fact, a further drop in body temperature is often experienced when blood is first removed for warming in the external apparatus. It would be desirable for these reasons to provide a method and apparatus for directly warming blood in situ, i.e., within the patient""s body.
Hyperthermia, a condition of abnormally high body temperature, may result from exposure to a hot environment, overexertion, or fever. Body core temperatures can range from 38xc2x0 C.-41xc2x0 C. due to fever and may be substantially higher in cases of exposure and overexertion. Like hypothermia, hyperthermia is a serious condition and can be fatal. Also like hypothermia, simple methods for treating hyperthermia, for example, immersion of the patient in a cool water bath or administration of cool fluids, have long been known. Generally, these simple methods for treating hyperthermia suffer from the same drawbacks and limited effectiveness as the simple hypothermia treatments noted above.
It would therefore be desirable to develop more effective methods for lowering the body temperature of hyperthermic patients. Furthermore, it is sometimes beneficial to induce an artificial low-temperature condition (induced hypothermia) within a patient by artificial cooling. This may be desirable, for example, to reduce a patient""s requirement for oxygen during surgery or during a condition of cardiovascular collapse.
To achieve these goals, methods have been used in which a patient""s blood is removed from his body, cooled in external cooling apparatus, and returned to his body. This external cooling suffers from the same disadvantages as the external warming previously described. External cooling requires cumbersome apparatus and the temporary removal of blood-entails some degree of risk to the patient. It would therefore be desirable to devise a method and apparatus for cooling blood within the patient""s body.
The present invention provides methods and apparatus for modifying and controlling a patient""s body temperature. According to the present invention, a catheter is inserted percutaneously into a blood vessel of the patient. By controlling the temperature of a portion of the catheter lying within the blood vessel, heat may be selectively transferred to or from blood flowing through the vessel. The patient""s body temperature may thereby be increased or decreased as desired. Some embodiments of apparatus suitable for practicing the present invention will provide means for treating hypothermia by warming a patient""s blood. Other embodiments will provide means for treating hyperthermia or inducing a desired condition of hypothermia by cooling the patient""s blood.
Because blood circulates rapidly through the vascular system, the beneficial effect of warming or cooling blood within the vessel will be quickly felt throughout the patient""s body. In situ modification of blood temperature is further advantageous in that blood is not removed from the patient. Additionally, no external pump is needed to circulate the blood. Injury to blood components from the pump is thereby eliminated. Furthermore, the required apparatus is much simpler, less cumbersome, and easier to use than the external blood warming or cooling apparatus previously known.
A catheter suitable for practicing the present invention will include means for warming or cooling at least a portion of the catheter inserted into the blood vessel. It is desirable that such a catheter have a relatively small cross-section so as not to unnecessarily impede blood flow through the vessel. On the other hand, a large heat transfer surface area will facilitate rapid heat transfer between the catheter and the blood. Structural features may therefore be included to increase the surface area of the temperature controlled region.